The present invention relates to a diffusion barrier layer and protective coating for turbine engine components that are exposed to high temperature, oxidation and corrosive environments. More particularly, the invention is directed to forming a non-metallic oxide or nitride diffusion barrier layer between a superalloy substrate and a protective coating for the substrate. The protective coating can be an environmental coating or a bond coat for a thermal barrier coating on the turbine engine component, and is formed by depositing at least one platinum group metal on the diffusion barrier layer.
In aircraft gas turbine engines, the turbine vanes and blades are typically made of nickel-based or cobalt-based superalloys that can operate at temperatures of up to about 1150° C. Various types of coatings are used to protect these superalloys. One type of protective coating is based on a material like MCrAl(X), where M is nickel, cobalt, or iron, or combinations thereof, and X is an element selected from the group consisting of Ta, Re, Ru, Pt, Si, B, C, Hf, Y and Zr, and combinations thereof. The MCrAl(X) coatings can be applied by many techniques, such as high velocity oxy-fuel (HVOF), plasma spray, or electron beam-physical vapor deposition (EB-PVD). Another type of protective coating is an aluminide material, such as nickel-aluminide. A platinum-aluminide coating can be applied, for example, by electroplating platinum onto the substrate, followed by a diffusion step, which is then followed by an aluminiding step, such as pack aluminiding. These types of coatings usually have relatively high aluminum content as compared to the superalloy substrates. The coatings often function as the primary protective layer (e.g., an environmental coating). As an alternative, these coatings can serve as bond layers for subsequently applied overlayers, e.g., thermal barrier coatings (TBCs).
When the protective coatings and substrates are exposed to a hot, oxidative, corrosive environment (as in the case of a gas turbine engine), various metallurgical processes occur. For example, an adherent alumina (Al2O3) layer (“scale”) usually forms on top of the protective coatings. This oxide scale is desirable because of the protection it provides to the underlying coating and substrate.
At elevated temperatures, interdiffusion of elemental components between the coating and the substrate often occurs. The interdiffusion can change the chemical characteristics of each of these regions, while also changing the characteristics of the oxide scale. In general, there is a tendency for the aluminum from the aluminum-rich protective layer to migrate inwardly toward the substrate. At the same time, traditional alloying elements in the substrate (e.g., a superalloy), such as cobalt, tungsten, chromium, rhenium, tantalum, molybdenum, and titanium, tend to migrate from the substrate into the coating. (These effects occur as a result of composition gradients between the substrate and the coating).
Aluminum diffusion into the substrate reduces the concentration of aluminum in the outer regions of the protective coatings. This reduction in concentration will reduce the ability of the outer region to regenerate the protective alumina layer. Moreover, the aluminum diffusion can result in the formation of a diffusion zone in an airfoil wall, which undesirably consumes a portion of the wall. Simultaneously, migration of the traditional alloying elements like molybdenum and tungsten from the substrate into the coating can also prevent the formation of an adequately protective alumina layer.
A diffusion barrier between the coating and the substrate alloy can prolong coating life by eliminating or greatly reducing the interdiffusion of elemental components. However, very thin layers of some materials may be insufficient for reducing the interdiffusion at high operating temperatures. Also, there should not be a substantial mismatch in CTE (coefficient of thermal expansion) between the protective coating, the diffusion barrier layer and a superalloy substrate. Otherwise, the overlying coating may spall during thermal cycling of the turbine engine component.
Thus, new diffusion barrier layers and protective coatings that overcome some of the drawbacks of the art would be welcome for high-temperature superalloy substrates. The barrier layer should have relatively low “interdiffusivity” for substrate elements and the protective coating. The barrier layer should also be chemically compatible and compositionally stable with the substrate alloy and any protective coating, especially during anticipated service lives at temperatures up to about 1150° C. Moreover, the barrier layer should exhibit a relatively high level of adhesion to both the substrate and the protective coating. The barrier layer should also exhibit only a minimum of CTE mismatch with the substrate and protective coating. Furthermore, the barrier layer and the protective coating should be capable of deposition by conventional techniques.